Seborrheic keratoses (SKs) are the most common benign epithelial tumors in humans. The etiology of SKs is unknown but they exhibit histologic evidence of increased proliferation of keratinocytes. These lesions have an increased rate of apoptosis and several studies show that their incidence increases with age. Some studies have found that 88% of individuals over age 64 have at least one SK.
SKs are characterized as a dull hyperkeratotic macule that evolves to a papulonodular lesion. They can appear as pale brown, pink, tan or brown in color and the surface can become warty or verrucous. The size varies from 5 mm to several centimeters and a classic “stuck on” appearance is observed. SKs almost never progress to malignant tumors.
SKs commonly harbor multiple oncogenic mutations in FGFR3, PIK3CA, KRAS, HRAS, EGFR, and AKT1 oncogenes but not in tumor suppressor genes p53, TSC1, and PTEN. There is no evidence indicating that a senescence program is activated in SKs. The expression profile of SKs is very similar to malignant skin tumors such as squamous cell carcinomas with the exception that SKs harbor a strong activation of a pro-differentiation program governed by a feedback loop between activated receptor tyrosine kinase signaling (such as FGFR3) and the transcription factor FOXN1.
Acanthosis nigricans (AN) is a brown, velvety plaque that usually arises symmetrically in flexural folds, such as in the axilla, around the neck, in the groin and under the breasts. Sporadic cases are due to several causes, but most commonly to insulin insensitivity and diabetes. Obese individuals are also at risk of developing AN. In addition, several internal malignancies may present clinically with AN.
Keratoacanthomas (KAs) are considered low-grade variants of cutaneous squamous cell carcinoma (SCC). Clinically, they are distinguishable from SCC by these features: (1) a “volcano-like” appearance with a thick keratin plug, (2) being itchy or painful, (3) spontaneous involution with scarring, (4) appearance in surgical scars or in traumatized skin and (5) inability to metastasize. However, histologically they closely resemble well-differentiated SCC and most pathologists are reluctant to diagnose lesions as KA. No special stains are available to aid the pathologist. As a result KAs are treated as if they were malignant and are almost always surgically excised, adding health risks and costs to treatment.
Epidermal nevi (EN) are benign lesions presenting at birth or in childhood. Epidermal nevi are overgrowths of structures and tissue of the epidermis, the outermost layer of the skin. The different types of epidermal nevi can vary in size, number, location, distribution and appearance.